Image credit: Maksim Borovkov
Discovering and understanding the world drives human beings. Simply observe a child mesmerised by the dust in a sunbeam. This same fascination is the essence of scientific endeavours; the emotions of the child are not so different from what Thompson must have felt when he discovered the electron, or when Millikan demonstrated the particle nature of the photon. Those two particles, the electron and the photon were later understood to be examples from two great families, fermions and bosons, to which all other particles in nature belong. Well, almost all. There is another possible category of particles: the so-called anyons. Anyons are predicted to emerge inside low dimensional materials from the collective dance of many interacting electrons. One important example are Majorana zero modes, anionic cousins to the Majorana fermions discovered theoretically by Ettore Majorana in 1937. Majoranas, as these hypothetical anyons are sometimes affectionally called, are predicted to exhibit numerous exotic properties, such as a nontrivial mutation of their mutual quantum phase as they move around each other, or a capability to hide quantum information by encoding it non-locally in space. These properties have triggered the imagination of many scientists for years, and have even been proposed as the key to robust quantum computers. Alas, there is no consensus on whether Majoranas have yet been detected in experiments.
Since 2010 many research groups have raced to find Majoranas. Unlike fundamental particles, such as the electron or the photon which naturally exist in vacuum, Majorana anyons need to be created inside hybrid materials. One of the most promising platforms for realising them is based on hybrid superconductor-semiconductor nanodevices. Over the past decade these devices have been studied with excruciating detail, with the hope to unambiguously prove the existence of Majoranas. However, Majoranas are tricky entities, easily overlooked or mistaken with other quantum states. Different special techniques have been developed, specially tailored to this task, and a number of potential detections have been reported. However, their interpretation remains steeped in controversy.
Together with Elsa, Ramon and Pablo from the Materials Science Institute in Madrid (Spanish Research Council CSIC) and Sara, Marc and Jordi from the Catalan Institute of Nanoscience and Nanotechnology we decided to shine further light into the mystery of Majorana physics by combining our expertise in electron microscopy, low temperature electrical measurements and condensed matter theory. Have Majoranas been detected or not? We applied two well-established techniques simultaneously to the same device. To our surprise, we found that the two techniques yielded contradicting conclusions. The states observed with one technique, highly suggestive of Majoranas at first glance, where not present when looking for them from the different perspective afforded by the second technique. We could thus conclude that no Majoranas existed in the first place.
The observations are akin to the following metaphorical scenario. In search of the fabled Majorana rock star, you peek through a crack of the door to a bar where a concert is taking place. You clearly see a remarkable chap on the stage, dressed in the Majorana outfit, singing the Majorana song. The bar is full of Majorana fans that watch him in adoration. The chap sure looks like Majorana! However, you astutely decide to check if everything is what it seems. You open a large door on the far end of the bar, so the fans begin to leave the premises little by little. To your dismay, you see how the rock star, finding himself alone, tosses his guitar and leaves. He turned out to be an attention-seeking impostor! The true Majorana would never do such a thing, he’s a true artist. That’s precisely what makes him special. Much like the true rock star will refuse to leave the stage, the Majorana anyon remains pinned to one side of the nanodevice by virtue of a profound mathematical principle called topological protection, even when regular electrons are allowed to escape through the opposite side.
This work highlights the fact that convincing Majorana impostors are everywhere. They can exist in many different types of devices, and are able to fool different measurement strategies individually. The combination of two measurement strategies applied to the same device revealed the impostor in our case through an apparent paradox, an approach that could drastically reduce interpretation ambiguities of future experiments. This is a much needed step so that one day we may trap the elusive Majorana, and finally begin to harness its power.